1. Field of the Invention
The present invention relates to a transmission for changing the transmission path of a torque by effecting the transmission of a torque to a predetermined rotary member or by intercepting the torque transmission and, more particularly, to an electrically controllable transmission.
2. Related Art
For example, a vehicular transmission is constructed such that the ratio in the number of revolutions between an input side rotary member and an output side rotary member is changed to execute speed changes: by selecting any of a plurality of paired gears of the constant-mesh type having different gear ratios; by changing a coupled state or a fixed state of rotary elements in a plurality of sets of planetary gear mechanisms; changing the external diameters of pulleys for the torque transmission; or by changing the contact position of a power roller, as arranged between a pair of discs, with the discs. The speed changes thus performed are executed not only by selecting the gear pair to participate in the torque transmission by transmitting the operating force of a shift lever to the transmission through a link mechanism but also by converting a gear ratio selecting operation into an electric signal thereby to activate a hydraulic actuator on the basis of the electric signal.
Moreover, an automatic transmission for selecting a gear stage automatically on the basis of the running state of a vehicle is constructed such that the gear stage is established according to the running state by detecting the vehicular running state electrically to convert it into an electric signal or by detecting the running state in terms of an oil pressure so that control valves may be interchanged on the basis of the electric signal or the hydraulic signal. This construction is disclosed, for example, in Japanese Patent Laid-Open No. 80847/1990 (JP-A-2-80847), in which the automatic transmission is integrally provided with a hydraulic servo mechanism in a frictional engagement device such as a clutch or a brake. Specifically, the frictional plates on the drive side and the frictional plates on the driven side are alternately arranged, and a piston to be reciprocally moved to and from those frictional plates is accommodated in a cylinder portion formed by a clutch drum or a casing. An oil pressure chamber is formed on the back side of the piston, and a return spring is arranged on the front side of the piston. Moreover, the piston is moved back and forth to apply/release the frictional engagement device by feeding/releasing the oil pressure to the oil pressure chamber via an oil passage formed through a shaft of rotation or the casing.
The hydraulic servo mechanism thus far described is employed as an actuator of not only the frictional engagement device of the automatic transmission but also the continuous variable transmission. In the prior art, moreover, there has also been developed a transmission which is enabled to have a manual shift mode similar to that of the manual transmission by outputting a speed change signal in the automatic transmission on the basis of the manual shifting operation so that the hydraulic servo mechanism may be controlled with the speed change signal. The reason why the oil pressure is adopted in any of those transmissions is that a force such as the operating force can be freely transmitted.
However, the transmission of this kind is constructed to change the portion to feed the oil pressure according to the control signal and to regulate the oil pressure. This construction requires a mechanism such as a hydraulic pump or a regulator valve for generating a line pressure as the source pressure. There are additionally required a solenoid valve for converting the electric signal into the hydraulic signal and a mechanism for regulating the oil pressure to be fed to the solenoid valve. It is further necessary to charge the transmission at all times with a large amount of oil (or fluid).
Thus according to the hydraulic device, the control of the speed change can be facilitated, but the mechanism for generating, regulating and controlling the oil pressure is complicated and large-sized. As a result, the transmission is increased in its whole weight and large-sized to deteriorate its mountability on the vehicle.